Platelet activation plays an important role in the development and course of cardiovascular disease. Platelet levels of phosphatidylinositol(3,4)bisphosphate (PtdIns(3,4)P2) are known to increase dramatically upon agonist stimulation in platelet aggregation-dependent manner. However, little is known about the regulation of PtdIns(3,4)P2 in platelets and its possible roles in platelet functions. We propose that the enzyme that cleaves D4 position of PtdIns(3,4)P2, inositol polyphosphate 4- phosphatase type I (INPP4A) regulates PtdIns(3,4)P2 levels and platelet function and will study this using a mouse model. We will employ radiation chimeric mice harboring a null mutation (weeble) in INPP4A restricted to cells of hematopoietic lineage to determine the role of INPP4A in platelet function. Our preliminary results indicate that platelets deficient in INPP4A generate higher levels of its substrate PtdIns(3,4)P2 after agonist stimulation, compared to the levels seen in wild type platelets. INPP4A deficient platelets also show increased platelet aggregation in response to thrombin. Radiation chimeras exhibited a prothrombotic phenotype in a mouse carotid artery injury model. We will extend these findings and study the role of INPP4A deficiency on platelet functions ex vivo (platelet aggregation, secretion, clot retraction, procoagulant activity) and in vivo utilizing hemostasis and thrombosis mouse models, while at the same time exposing pharmacy students to basic research. Two students per year will be recruited to work with Dr. Marjanovic on the projects outlined in this proposal. They will work part time during the academic school year (5-8 hrs/week) and full time during summer breaks. Our study should define the role of INPP4A and its substrate PtdIns(3,4)P2 in platelet functions, which is of importance for better understanding of mechanisms that regulate hemostasis and thrombosis.